Reuse machine ID when re-using a cached image

Reuse machine ID when re-using a cached image

4fb53aea9e04b931cccedf1545831e426d6ec2f7 broke incremental builds
for bootable Fedora images because when building the cached image,
the initrd is installed to "/efi/<machine-id>/...", and when doing
a cached build, a new machine ID is generated inside mkosi (but not
written to /etc/machine-id), making gen_kernel_images() unable to
find the initrd.

Let's fix the issue by re-using the machine ID from /etc/machine-id
when doing a cached build.

In the future, we can use the new entry-token feature of bootctl to
have kernel-install write to directory in /efi that's not tied to
the machine ID but until that's widely available this will have to
do.